What qualities should a good pianist possess ?

It will depend on the type of music being played. It all comes down to an ability to express the music. For a solo clical pianist, this will mean the ability to play dynamics (different volume levels), technical skill, the ability to interpret the composition through phrasing, rubato, and historical recordings and literature, etc. For a pianist in a rock band, you will want to have fresh, creative ideas that can still garner audience appreciation and the ability to interweave the ideas with fellow band members' playing. You should also be proficient in multiple chord voicings and have the ability to play well-timed lines that cut through the band. These are just a couple of examples.